mypy: Clean test_subs.py of mypy strict-optional errors.

Most of this is just asserting that the sub_dict return value from
access_stream_by_id is not None in the cases where it shouldn't be,
but additionally, we also need to pass a function into
validate_user_access_to_subscribers_helper (in this case, just `lambda:
True` works fine)

mypy.ini

@@ -371,8 +371,6 @@ strict_optional = False
 strict_optional = False
 [mypy-zerver/tests/test_auth_backends]  #2079: error: Incompatible types in assignment (expression has type "Optional[UserProfile]", variable has type "UserProfile")
 strict_optional = False
-[mypy-zerver/tests/test_subs]  #2916: error: Item "None" of "Optional[Subscription]" has no attribute "recipient"
-strict_optional = False
 [mypy-zerver/tests/test_signup]  #1954: error: Item "None" of "Optional[Match[Any]]" has no attribute "groups"
 strict_optional = False
 [mypy-zerver/tests/test_realm]  #191: error: Item "None" of "Optional[Stream]" has no attribute "id"

zerver/tests/test_subs.py

@@ -807,7 +807,7 @@ class StreamAdminTest(ZulipTestCase):
     def attempt_unsubscribe_of_principal(self, query_count: int, is_admin: bool=False,
                                          is_subbed: bool=True, invite_only: bool=False,
                                          other_user_subbed: bool=True,
-                                         other_sub_users: List[UserProfile]=None) -> HttpResponse:
+                                         other_sub_users: Optional[List[UserProfile]]=None) -> HttpResponse:
 
         # Set up the main user, who is in most cases an admin.
         if is_admin:
@@ -2659,11 +2659,11 @@ class SubscriptionAPITest(ZulipTestCase):
 
         # This should result in missing user
         with self.assertRaises(ValidationError):
-            validate_user_access_to_subscribers_helper(None, stream_dict, None)
+            validate_user_access_to_subscribers_helper(None, stream_dict, lambda: True)
 
         # This should result in user not in realm
         with self.assertRaises(ValidationError):
-            validate_user_access_to_subscribers_helper(user_profile, stream_dict, None)
+            validate_user_access_to_subscribers_helper(user_profile, stream_dict, lambda: True)
 
 class GetPublicStreamsTest(ZulipTestCase):
 
@@ -3190,6 +3190,7 @@ class AccessStreamTest(ZulipTestCase):
         # Hamlet can access the private stream
         (stream_ret, rec_ret, sub_ret) = access_stream_by_id(hamlet, stream.id)
         self.assertEqual(stream.id, stream_ret.id)
+        assert sub_ret is not None
         self.assertEqual(sub_ret.recipient, rec_ret)
         self.assertEqual(sub_ret.recipient.type_id, stream.id)
         (stream_ret2, rec_ret2, sub_ret2) = access_stream_by_name(hamlet, stream.name)
@@ -3251,6 +3252,7 @@ class AccessStreamTest(ZulipTestCase):
         # Guest user have access to subscribed public streams
         self.subscribe(guest_user_profile, stream_name)
         (stream_ret, rec_ret, sub_ret) = access_stream_by_id(guest_user_profile, stream.id)
+        assert sub_ret is not None
         self.assertEqual(stream.id, stream_ret.id)
         self.assertEqual(sub_ret.recipient, rec_ret)
         self.assertEqual(sub_ret.recipient.type_id, stream.id)
@@ -3264,6 +3266,7 @@ class AccessStreamTest(ZulipTestCase):
         # Guest user have access to subscribed private streams
         self.subscribe(guest_user_profile, stream_name)
         (stream_ret, rec_ret, sub_ret) = access_stream_by_id(guest_user_profile, stream.id)
+        assert sub_ret is not None
         self.assertEqual(stream.id, stream_ret.id)
         self.assertEqual(sub_ret.recipient, rec_ret)
         self.assertEqual(sub_ret.recipient.type_id, stream.id)